Photoactive devices are semiconductor devices that employ semiconductor material to convert electromagnetic radiation into electrical energy or to convert electrical energy into electromagnetic radiation. Photoactive devices include, for example, photovoltaic cells, photosensors, light-emitting diodes, and laser diodes.
Photovoltaic cells (also referred to in the art as “solar cells” or “photoelectric cells”) are used to convert energy from light (e.g., sunlight) into electricity. Photovoltaic cells generally include one or more pn junctions, and can be manufactured using conventional semiconductor materials, such as silicon, germanium, and III-V semiconductor materials. Photons from impinging electromagnetic radiation (e.g., light) are absorbed by the semiconductor material proximate the pn junction, resulting in the generation of electron-hole pairs. The electrons and holes generated by the impinging radiation are driven in opposite directions by a built-in electric field across the pn junction, resulting in a voltage between the n region and the p region on opposing sides of the pn junction. This voltage may be used to produce electricity. Defects in the crystal lattices of the semiconductor materials at the pn junctions provide locations at which electrons and holes previously generated by absorption of radiation can recombine, thereby reducing the efficiency by which the radiation is converted into electricity by the photovoltaic cells.
The photons of the electromagnetic radiation that impinge on a photovoltaic cell must have sufficient energy to overcome the bandgap energy of the semiconductor material to generate an electron-hole pair. Thus, the efficiency of the photovoltaic cell is dependent upon the percentage of the impinging photons that have an energy corresponding to the bandgap energy of the semiconductor material. Stated another way, the efficiency of the photovoltaic cell is at least partially dependent upon the relationship between the wavelength or wavelengths of the radiation impinging on the photovoltaic cell and the bandgap energy of the semiconductor material. Sunlight is emitted over a range of wavelengths. As a result, photovoltaic cells have been developed that include more than one pn junction, wherein each pn junction comprises semiconductor material having a different bandgap energy so as to capture light at different wavelengths and increase the efficiencies of the photovoltaic cells. Such photovoltaic cells are referred to as “multi-junction” or “MJ” photovoltaic cells.
Thus, the efficiency of a multi junction photovoltaic cell may be increased by selecting the semiconductor materials at the pn junctions to have band-gap energies that are aligned with the wavelengths of light corresponding to the wavelengths of highest intensity in the light to be absorbed by the photovoltaic cells, and by decreasing the concentration of defects in the crystal lattices of the semiconductor materials at the pn junctions. One way to decrease the concentration of defects in the crystal lattices of the semiconductor materials is to employ semiconductor materials that have lattice constants and coefficients of thermal expansion that are closely matched with one another.
Previously known multi junction photovoltaic cells are relatively inefficient in conversion of electromagnetic radiation at wavelengths in the range extending from about 1,550 nm to about 1,800 nm. For example, it is known to employ a pn junction in a germanium (Ge) cell in a multi junction photovoltaic cell. As disclosed in, for example, M. Yamaguchi et al., Multi-junction III-V solar cells: current status and future potential, Solar Energy 79, pp. 78-85 (2005), and D. Aiken et al., Temperature Dependent Spectral Response Measurements for III-V Multi-Junction Solar Cells, Emcore Photovoltaics, 10420 Research Rd. SE, Albuquerque, N. Mex. 87123, the external quantum efficiency of such multi junction photovoltaic cells drops for wavelengths longer than about 1,650 nm. Without being bound to any particular theory, it is currently believed that this drop in external quantum efficiency is at least partially due to the fact that optical coupling between the photons of such radiation wavelengths and electrons in the Ge crystal lattice in the Ge cell involves an indirect electronic transition between the conduction band and the valence band. In addition to the photon and electron, the optical coupling process requires a phonon to conserve momentum. Due to the requirement of the phonon to conserve momentum, the indirect electronic transition process leads to a low optical absorption coefficient for photons having wavelengths greater than about 1,650 nm, and such photons are likely to be absorbed only after passing through a sufficient physical thickness of Ge.
Additionally, previously known Ge solar cells often include Ge formed on a heavily doped p-type substrate. As a result, the minority carrier (electron) diffusion length within the Ge is shorter than the actual physical thickness of the Ge layer in which the pn junction is foamed. As a result, most photons having wavelengths greater than about 1,650 nm do not generate electrons that can diffuse a sufficient distance to an electrode prior to undesirable recombination with an electron hole, and thus cannot contribute to the photocurrent of the photovoltaic cell.